Method for controlling peroxide bleaching in a plurality of bleaching stages

ABSTRACT

Method of peroxide bleaching of mechanical, thermomechanical and chemi-mechanical pulp wherein the peroxide bleaching is controlled by addition of a known amount of bleaching chemicals in the first stage which amount is allowed to react under defined conditions whereafter the brightness of the pulp after this first stage is used for control of a subsequent stage. In the first stage fresh chemicals, chemicals recirculated from a subsequent bleaching stage or a mixture of these is used. Hydrogen peroxide is the preferred bleaching agent but other peroxides can also be used.

The invention relates to a method of controlling peroxide bleaching ofmechanical, thermomechancial or chemi-mechanical pulp.

For several products, such as soft tissue, paperboard and differenttypes of fine paper, it has started to become more and more common touse bleached mechanical or chemi-mechanical pulps instead of fullybleached chemical pulps. Besides the fact that the production ofmechanical pulp is much more attractive from an environmental point ofview than the production of chemical pulp, the raw materials are alsomore efficiently utilized. This means that mechanical pulp can beproduced at a considerably lower cost and, in several aspects,mechanical pulp also has better properties than the chemical pulp.However, up to now a disadvantage of the mechanical pulp has been alower brightness which has limited its use in several types of products.

As a consequence of the development of the peroxide bleaching process,for example by beaching in several stages and at high pulpconcentrations, it has been possible to increase the brightness and atthe same time reduce the costs for chemicals. Previous bleachingsystems, both one and two stage systems, have, however, shown aconsiderable disadvantage in that the possibilities of controlling,regulating and optimizing the bleaching have been limited.

In existing bleaching plants the control is in the simplest case basedon measurement of the brightness of the incoming pulp and the brightnessvalue is then used directly for adjustment of the addition of bleachingchemicals. According to another system, which is more common, thebrightness of the pulp is measured after the addition of the chemicalsand after a defined reaction time of between 1 and 5 minutes. Thebrightness value is then used for "feed-back " regulation of theaddition of the chemicals.

The brightness of the unbleached pulp is, however, not a satisfactorymeasure of the bleachability of the pulps and changes in the brightnesscan depend on several factors which influence the relation between thechemical addition and the brightness of the finished pulp in variousways. The raw material can thus vary with regard to content of rottenmaterial, storage time, bark content and blends of different types ofwood. The process conditions vary with the blends of chemicals,differences in degree of beating, the temperature and the treatmenttimes and these and other factors influence the relation between theaddition of chemicals and the brightness of the finished pulp indifferent ways.

The present invention will now be disclosed in more detail withreference to the appended drawings.

FIG. 1 shows the brightness of pulp at bleaching according to apreviously known method.

FIG. 2 shows the brightness of pulp at bleaching according to thepresent invention.

FIG. 3 shows the control of a peroxide bleaching system in two stagesaccording to the invention.

In FIG. 1 the brightness of pulps bleached in laboratory is shown as afunction of the brightness of the unbleached pulp. The peroxide additionhas in all cases been 40 kg/t H₂ O₂ and the addition of alkali has beenoptimized. The bleaching has been carried out on pulps produced indifferent manners, TMP, CTMP and groundwood pulp, and from differenttypes of wood, birch, aspen, eucalyptus, spruce and pine wood. All thepulps were bleached under identical conditions and the poor correlationbetween unbleached and bleached brightness is clearly evident.

In closed systems, for example groundwood mills and TMP-plants, whereinthe white water from the bleaching plant is used for dilution after thedefibration the brightness of the incoming pulp will of course be aneven poorer basis for the control. The brightness of incoming materialto the bleaching plant will in these cases be strongly dependent on theamount of residual bleaching chemicals which are recycled with the whitewater and this residual amount is in turn set by the degree of systemclsorue and the amount of residual chemicals from the bleaching. Anincreased brightness in the feed material to the bleaching plant in sucha system does not necessarily mean that the bleachability of the pulphase been improved, but only that a somewhat greater part of the first"simple "part of the bleaching has already been carried out by theresidual chemicals.

A system with measurement of brightness after a certain reaction time,and "feed-back "-regulation of the addition of chemicals will thus bemore or less unusable in feed-back systems which has been clearlyevident in real operation. Such a regulation will be completelymisleading particularly at production changes, starts, stops, etc. whenthe chemical balance in the system is altered drastically.

The object of the present invention is to achieve a perfectlysatifactory control of peroxide bleaching both when the incoming rawmaterials very as when recycled chemicals from the bleaching are usedfor bleaching the pulp before the bleaching plant. The control ofbleaching according to the invention means that excess use of bleachingchemicals can be avoided and considerable savings in bleaching chemicalshave been made in actual practice of the present method. Another veryimportant advantage is that flutuations due to factors stated above areavoided and the brightness of the outcoming material from the bleachingplant is very even which is of the greatest importance for the producer.

The control of bleaching according to the invention is directed toperoxide bleaching in more than one stage. The method is particularlyapplicable to bleaching with hydrogen peroxide, but can also be used forbleaching with other known peroxide bleaching agents for pulps, such assodium peroxide and sodium percarbonate. Hydrogen peroxide bleaching iscarried out in alkaline solution, usually within a pH range of from 6 to12, and generally with hydrogen peroxide amounts of from 0.1 to 10 percent by weight based on dry pulp. The pH is adjusted with alkalineagents, mainly caustic soda and water glass. According to knowntechnique chelating agents such as EDTA and DTPA are used to eliminatethe influence of contaminating metals.

The method of the invention is particularly applicable to two-stagebleaching plants where, in existing systems, the first stage is mainlyused for a "passive "consumption of the chemicals remaining from thesecond bleaching stage. According to the invention the first stage isinstead used "actively "for determination of the bleachability of thepulp. A known amount of chemicals is added to the first stage and isallowed to react under known conditions. The brightness from the firststage is then directly used for control of the conditions "feed-forward", and mainly for the addition of chemcials in subsequent bleachingstages. The known amount of chemicals can be freshly added chemicals,recovered unreacted chemicals from subsequent stages or, which is mostoften the case, a mixture of these two types. The known amount ofchemicals is alolowed to react under known conditions with regard to pH,temperature, time and pulp concentration. From practical experience ithas been found that the freshly added bleaching chemicals to the firststage suitably should be from 5 to 60 per cent by weight of the totallyadded amount. In some cases it has been found that the amount ofbleaching chemicals can be entirely covered by recycled chemicals.Alkali is usually added in this stage in an amoount corresponding to 20to 60 or up to 80 per cent by weight of the total addition for thebleaching sequence. Besides the main use of the measured brightnessafter the first stage for control of "feed-forward "conditions, thelevel of brightness after the first stage can also be used foradjustment of the addition to the first stage, so that an optimumdistribution of the chemical addition between the stages and thedevelopment of brightness over the stages is obtained.

In FIG. 2 is shown the brightness of pulp bleached with 40 kg/t ofhydrogen peroxide as a function of the brightness of the same pulpbleached with 20 kg/t of hydrogen peroxide. The alkali addition isoptimized and in the same manner as in FIG. 1 different types of woodand different processes have been used. The brightness of the pulp afterthe finished bleaching with 40 kg of hydrogen peroxide per ton has beenset against the brightness for the same pulp bleached with half theamount of chemical, 20 kg of hydrogen peroxide per ton. As evident fromthe figure the correlation is very good, and, further, in principleindependent of both process and wood raw material, i.e. in totalcontrast to what is shown in FIG. 1.

Several runs have been made wherein the addition in stage two has beenadjusted according to the brightness values from stage 1. Even at loweradditions in stage 1, in the range of from 10 to 20% of the entireaddition, a good correlation between the brightness of the finishedbleached pulp and the value from stage one is obtained.

This good correlation is direct proof that the bleach results from afirst bleaching stage which has been run under known conditions can beused directly for control of a subsequent stage, particularly in thosecases where the aim is to achieve high brightness levels for the finalbleached pulp.

In FIG. 3 an embodiment for control of a peroxide bleaching system intwo stages is shown. The two-stage bleaching plant is integrated in aline for production of bleached market pulp. The production of the pulpbefore the bleaching plant can be mechanical, SGW, TMP, RMR, (StoneGround Wood, Thermo Mechanical Pulp, Refiner Mechanical Pulp) etc, orchemi-mechanical, CTMP, CMP, NSSC, (Chemi-Thermo Mechanical Pulp,Chemical Mechanical Pulp, Neutral Sulphite Semi Chemical) etc.

The incoming pulp 1 is thickened in the press 2 to a pulp concentrationof about 33%, mixed with bleaching chemicals 3 in the mixer 4 andbleached in the bleaching tower 5 of the first stage at a pulpconcentration of about 10%. The bleached pulp is thickened to about 33%in the press 6 and the bleaching chemicals 7 for the second stage arethen added in mixer 8. The pulp from the bleaching tower 9 of the secondstage is diluted in the screw 10 and the pulp chest 11 and thickened inthe press 12. The thickened pulp which has a dry solids content of about50% is brought from the press to the storage tower 13 of the drier. Therecovered, chemical-containing, white water from the press 12 iscollected in a white water tank 14 and reused for dilutions after thebleaching tower. Excess of white water is reused in the first bleachingstage after required addition of fresh chemicals in the tank 15 forcorreciton of the dosage of chemicals to the first bleaching stage.

At bleaching according to the invention the control is made throughmeasuring of different parameters in the production line and input ofsignals from the sensor to a computer which give control signals todifferent valves about regulators etc. The control system is shown inFIG. 3.

The production is determined by measuring pulp flow 20 and pulpconcentration 21 up to the first stage. The production signals are usedfor regulation of the chemical flows in dependence of the production.The temperature 22 of the incoming pulp to stage 1 is measured and canbe adjusted by steam addition 23. The level 24 in the tower 5 is used asa measure of the bleaching time. The bleaching results are continuouslymeasured with a brightness meter 25 and the brightness value is used forregulation of a chemical addition to stage 2 and optionally forfeed-back-regulation of the chemical addition to stage 1. The level ofthe white water tank is regulated 26 and the bleaching conditions instage 1 are controlled by continuous measurement of pH 27 and residualperoxide 28 in the white water from the press 6 after the bleachingstage. The concentration of the pulp to the press 6 is controlled 29 byaddition of white water. A flow 30, corresponding approximately to thebalanced white water excess from stage 2, is used for the chemicaladdition in stage 1. The addition of fresh chemicals to stage 1 isregulated by the valves 31-34. DTPA 31 and sodium silicate 32 are addedaccording to a set value in proportion to the production. The additionof fresh alkai 33 and peroxide 34 is adjusted with regard to the amountof alkali and residual peroxide in recycled white water measured with 35and 36. The white water dilution to the mixing tank 15 is controlled by37.

For the incoming pulp to stage 2 the temperature 38 is measured and canbe adjusted by addition of steam 39. The level 40 is used as ameasurement of the bleaching time. The bleach results of the pulp fromstage 2 is controlled by brightness measurement 41. In the white watertank 14 the level 42 is regulated and at a too low level the tank isfilled with warm water. At a too high level the excess of white water ispumped to the screen room 43. The level is balanced with regard to thevolume taken out via 37. For control of the bleaching conditions instage 2 the pH 35 and the peroxide content 36 in the white water fromthe press after the bleaching stage are continuously measured. Thesignals are also used for adjustment of the chemical additions to stage1.

The conecentration regulation 44 of the pulp at the press 12 is madewith white water from the press. The added amount of warm water 45 aswash water to stage 2 is selected with regard to the type of pulpproduced and is set at a ratio to the production. The bleach liquid tostage 2 consists of a chemical solution diluted with water to avoiddecomposition of the peroxide. The flow 46 is proportioned to theproduction. The composition is regulated by the meters 47, 48, 49 forperoxide, alkali and silicate, respectively. The addition is controlledby the bleachability, i.e. the brightness value from 25 with regard tothe preoxide addition 34, the time 24, residual peroxide 28 andtemperature 22 in stage 1 and proportioned to the production. A freshwater flow 50 is brought to the mixing tank for the chemicals. Theoutflow of the pulp from stage 2 is controlled by the regulator 51.

In practice it has been found that by control of the bleaching accordingto the invention the disadvantages of previous control methods areavoided and that an even and uniformly bleached pulp can be producedindependent of variations in the raw material and/or the production. Thedisclosed embodiment can of course also be varied, within the scope ofthe invention, by the man skilled in the art for adaption to differentplants. In the following example a typical bleaching operaion using thecontrol system of the present invention is shown.

EXAMPLE

The control system was tried out in a CTMP mill producing pulp bleachedin two stages using hydrogen peroxide. The pulp type was fluff with afreeness of about 600 CSF anf the target brightness was 76% ISO. The rawmaterial was Scandinavian spruce with some pine admixture, less than20%. The initial brightness before bleaching was 60 plus minus 0.5% ISOduring the whole run.

The first bleaching stage was set to be run with a constant peroxidecharge of 15 kg/ton of pulp. This was decided based on laboratoryexperiments giving a curve showing the amount of peroxide required toreach 76% ISO in stage two as a function of brightness in stage one whenthe charge in this was 15 kg/ton of pulp. This curve will in thefollowing be referred to as alogorithm-15. It should be pointed out thatalgorithms have to be made up for each specific pulp and peroxide chargein stage one, raw material and final brightness target. This can be donein the laboratory or in the mill, e.g. with the aid of a computer.

The volumetric flow of spent liquor recycled from stage two wascontinuously monitored as was its content of residual peroxide.

At the start of the bleaching the amount of recirculated peroxide wasobviously nil and thus the freshly added amount was 15 kg/ton. As thebleaching continued, the content of peroxide in the stream of spentliquor from stage two began to rise and consequently the freshly addedamount was reduced so that the total charge to stage one was keptconstant.

The brightness after stage one was also monitored continuously and thefigure entered into algorithm-15 which delivered a target figure for therequired total peroxide dosage in stage two. Also in stage two the totaladded peroxide is made up of freshly added chemical plu carry-over fromstage one.

It was found that the brightness level of the finished pulp was withinplus minus 0.5 ISO units from the required 76% ISO during the wholetrial period which was one week. The value of the present method wasthus amply demosnstrated.

The mill where the bleaching was run uses several wood suppliers and thechips are of different quality due to different storage and transporttimes etc. In the first two days of the run, the bleaching response instage one turned out to be that 15 kg/ton of peroxide gave a brightnessof 66% ISO which, in accordance with algorithm-15, required another 25kg/ton in stage two. On the third day different quality chips were fedinto the plant and bleaching response fell from 66 to 64% ISO afterstage one. The algorithm-15 then prescribed 28.5 kg/ton of bleachingagent. Dosage in stage two was accordingly changed and final brightnesswas maintained at 76% ISO without interruption.

If the brightness response in stage one had not been detectedimmediately and correction in stage two not undertaken, then thebrightness of the finished pulp would have been below target and thetime elapsed before the plant could produce fully bleached grade wouldat least have been the holding time in stage two, in this case threehours. It should be pointed out that the initial brightness of theunbleached pulp did not change when the raw material was altered.

We claim:
 1. A method for controlling peroxide bleaching of mechanical,thermomechanical and chemi-mechanical pulp in a plurality of bleachingstages, comprising the steps of bleaching the pulp in a first stage witha predetermined amount of bleaching chemicals containing peroxide andunder predetermined reaction conditions, measuring the brightness of thepulp from this first stage and calculating the bleachability of the pulpas a function of the measured brightness and the predetermined amount ofchemicals and predetermined reaction conditions, feedforwardly adjustingthe amount of bleaching chemicals containing peroxide added in a secondstage as a function of the calculated bleachability of the pulp from thefirst stage, and bleaching the pulp in the second stage.
 2. A methodaccording to claim 1, wherein the bleaching chemcials in the first stageare selected from the group consisting of fresh chemicals, chemicalsrecirculated from a subsequent bleaching stage, and mixtures thereof. 3.A method according to claim 1, wherein the addition of peroxide to thesecond stage is from about 40 to 100 percent weight of the totaladdition of peroxide.
 4. A method according to claim 2, wherein theaddition of peroxide to the second stage is from about 40 to 100 percentweight of the total addition of peroxide.
 5. A method according to claim1, wherein the bleaching chemcials in the first stage include whitewater recirculated from a subsequent bleaching, and wherein an amount ofperoxide is added to the bleaching chemicals in the first stage as afunction of the amount of peroxide in the white water.
 6. A methodaccording to claim 2, wherein the bleaching chemicals in the first stageinclude white water recirculated from a subsequent bleaching and whereinan amount of peroxide is added to the bleaching chemicals in the firststage as a function of the amount of peroxide in the white water.
 7. Amethod according to claim 3, wherein the bleaching chemicals in thefirst stage include white water recirculated from a subsequent bleachingand wherein an amount of peroxide is added to the bleaching chemcials inthe first stage as a function of the amount of peroxide in the whitewater.
 8. A method according to claim 4, wherein the bleaching chemcialsin the first stage include white water recirculated from a subsequentbleaching and wherein an amount of peroxide is added to the bleachingchemicals in the first stage as a function of the amount of peroxide inthe white water.
 9. A method according to claim 1, wherein the bleachingchemicals in the first stage include white water recirculated from asubsequent bleaching stage, the white water including alkali, andwherein an amount of alkali is added to the bleaching chemicals in thefirst stage as a function of the amount of alkali in the white water.10. A method according to claim 2, wherein the bleaching chemicals inthe first stage include white water recirculated from a subsequwentbleaching stage, the white water including alkali, and wherein an amountof alkali is added to the bleaching chemcials in the first stage as afunction of the amount of alkali in the white water.
 11. A methodaccording to claim 3, wherein the bleaching chemicals in the first stageinclude white water recirculated from a subsequent bleaching stage, thewhite water including alkali, and wherein an amount of alkali is addedto the bleaching chemicals in the first stage as a function of theamount of alkali in the white water.
 12. A method according to claim 4,wherein the bleaching chemicals in the first stage include white waterrecirculated froma subsequent bleaching stage, the white water includingalkali, and wherein an amount of alkali is added to the bleachingchemicals in the first stage as a function of the amount of alakli inthe white water.
 13. A method according to claim 1, wherein the methodincludes recovery of white water from the second stage, and wherein fromabout 40 to 100 percent by weight of the recovered white water isrecirculated to the first stage.
 14. A method according to claim 2,wherein the method includes recovery of white water from the secondstage, and wherein from about 40 to 100 percent by weight of therecovered white water is recirculated to the first stage.
 15. A methodaccording to claim 3, wherein the method includes recovery of whitewater from the second stage, and wherein from about 40 to 100 percent byweight of the recovered white water is recirculated to the first stage.16. A method according to claim 4, wherein the method includes recoveryof white water from the second stage, and wherein from about 40 to 100percent by weight of the recovered white water is recirculated to thefirst stage.
 17. A method according to claim 1, wherein the peroxide ishydrogen peroxide.
 18. A method according to claim 2, wherein theperoxide is hydrogen peroxide.
 19. A method according to claim 3,wherein the peroxide is hydrogen peroxide.
 20. A method according toclaim 4, wherein the peroxide is hydrogen peroxide.
 21. A methodaccording to claim 5, wherein the peroxide is hydrogen peroxide.